7D Auto ISO vs. fixed ISO?

Started Apr 6, 2014 | Questions thread
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fpix Regular Member • Posts: 311
Re: 7D Auto ISO vs. fixed ISO?

RGF wrote:


So many opinions here... Covering so many beliefs... Should I try mine? I will give it a try.

AutoISO is just an automated method to select proper parameters A/T@ISO by the camera. That means one needs to properly understand how this works before relying on it and using it "all the time". This is not trivial (but also no rocketscience). One needs to play with it and understand it before using it. If additional to this automatics the photographer uses evaluative metering, which is another automatic procedure - even harder to control, and if he does not properly understand this either, then there will be a lot of things beyound control and results can be seen in above image.

Regarding above picture, T is absolutely fine. EXIF says F = 260mm, thus maximum T should have been roughly 1/260/1.6 = 1/400s, assuming you know to avoid camera shake. T in EXIF is 1/800s. Subject is not moving fast, thus longer T would not have been a problem. This means you, RGF, may have used lower ISO actuallly. With static subjects or if you are carefull to catch the moment when subject stays still --> even longer T may have been used, observing that you have IS on your lens. Thus lower ISO would have been possible in the given situation.

Do not understand me wrong. Short T is always good for sharp images. But if there is no light then you need to make a compromise somewhere with given equipment and situation.

Regarding above picture - aperture, I see f/5,6. With rigorous control you might have had f/5.0, thus saving one more 1/3 stop in ISO. Could be a consequence of AutoISO if you were in Tv mode.

Regarding ISO - this is the one chosen by camera. Just one comment here. You said:
"... I used M, AV, and TV modes". You should observe AutoISO works differently with M vs. Av vs. Tv. I mean you may land on different A/T@ISO in Av vs. Tv vs. M and same given scene, even if all these exposures may be equal.

Now main problem. This was underexposure and metering is to be blamed for this. You, RGF, probably had evaluative metering with above picture and did not apply EC at all or not enogh. Check the hystogram. This image is heavily underexposed! Additional to this the subject is dark toned and ambient light is miserable. Underexposure at any ISO with 7D (2009 sensor) is bad. Underexposure at high ISO with 7D is catastrophic, just as your image shows. You may replicate the situation at home with camera on tripod and with perfect CDAF or manual focus. And you may observe what happens when you underexpose 2 stops or more at high ISO. You will get similar apparently unsharp images. Guess from where the "blurriness" comes from?

One more note. You may get "sharp" images in low light with high ISO when photgraphying subjects with few details, such as a Canon lens cap. Just because the eyes (and brain) of the viewer does not expect to see many fine details in the lens cap. But when the subject is a bird the viewer expects to see details in the feathers. High ISO on 7D is not an option anymore, especially if combined with underexposure.

So quite many problems here, not mentioning processing (in camera? RAW & postprocessing? how competent processing?), crop? bad light, bad scene - dark subject with light surroundings, composition, subject size, a.o..

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